Ethylene-based polymers, such as high-pressure radical polymerization process polyethylene, linear low density polyethylene, and high density polyethylene, have been processed by various extrusion processes into various types of molded articles, such as films, sheets, hollow containers, pipes, tubes, resin-coated electric wires, resin-coated steel materials, and foamed articles.
In extrusion of ethylene-based polymers, various additives are usually used together according to the intended application. When such an additive is a liquid, there is known a method in which an ethylene-based polymer is kneaded in an extruder and the liquid additive is added with a pump or a feeder during the kneading. However, such a method needs specially-designed equipment, such as pump or feeder, for adding a liquid additive. In the case that the additive is caused to react with a resin, the length of an extruder and the position where the additive can be added are restricted.
Although a method in which both a liquid additive and an ethylene-based polymer are fed into a hopper and kneaded in an extruder has also been known, in the case that a conventional pellet is used as an ethylene-α-olefin copolymer and kneaded in a single-screw extruder, extrusion may be unstable due to slipping of the wet pellet. As a measure for this, a method using an ethylene-α-olefin copolymer in a form other than pellet is investigated. Patent document 1 has disclosed a method of producing an electric wire and cable extruded using a granular linear low density polyethylene and a linear low density polyethylene pellet. Patent document 2 has disclosed a method of obtaining a water-crosslinked molded article using a granular linear low density ethylene-α-olefin copolymer.    Patent document 1: JP 7-130238 A    Patent document 2: JP 8-134147 A
In both patent documents 1 and 2, an ethylene-α-olefin copolymer in a shape other than pellet is used, and there is not known any method in which extrusion process is stably conducted by using a single-screw extruder using a pellet, which has been used widely in the form of ethylene-based polymers, and a liquid.
Under such a situation, one problem to be solved by the present invention is to provide an extruded article that is made of a highly-crosslinked ethylene-based polymer and that is extruded stably with a single-screw extruder under direct addition of a liquid to a pellet without addition such a liquid to a molten resin by using a feeder, a pump, or the like. Another problem is to provide a method of conducting extrusion process by the use of a single-screw extruder using that pellet and adding a liquid additive.